Metals may be attacked by corrodants that are present in the environments in which the metals operate. For example, aluminum articles contacted to a salt-containing environment may be attacked at their surfaces either generally over a large area or locally in limited areas, for example at weld joints, at bolt holes, or at small inclusions or pits in the surface. The corrosion damage increases over time and with continued exposure, eventually possibly leading to such severe corrosion that there is a premature initiation of failure of the article at an earlier time than would otherwise be the case in the absence of the corrosion damage. Large amounts of money are spent on the corrosion protection, yet corrosion damage and corrosion-induced premature failure are still widespread.
Coatings are widely employed to protect surfaces against corrosion damage. Some of the most effective coatings employ hexavalent chromium having chromium atoms in the +6 oxidation state (Cr+6), usually in the form of chromate ions CrO4−2 or dichromate ions Cr2O7=, as part of the coatings to impact corrosion resistance to the surfaces. Chromate conversion coatings chemically bond strongly to the surfaces of the articles when exposed at room temperature, and thereafter inhibit corrosion at the surfaces. Surfaces passivated by reaction with chromate tend to slow the rate of oxygen reduction, which drives the corrosion reaction. By being effective at very low concentrations, chromate is one of the most effective universal corrosion inhibitors.
There is a desire to reduce the amount of chromate used in coatings and other applications, largely because hexavalent chromium ions can have adverse environmental effects and adverse health effects. Future regulations are expected to impose large reductions in the amount of hexavalent chromate that may be used in most applications, including coatings for reducing the corrosion of articles.
At the present time, there are no effective substitutes for the chromate-containing coatings. Some non-chromate coatings are available to improve the adhesion of paint primers and paints to surfaces, but the non-chromate coatings themselves have less corrosion-resistance than those containing chromate. Other non-chromate coatings serve only as barriers between a corrosive medium and the surface of the underlying metal, without serving as active corrosion inhibitors. If the barrier of these coatings is breached, as for example by a hole or scratch extending through the barrier coating, there is no chemical inhibition of the resulting potential corrosion.
There is a need for an improved coating approach to protecting articles against corrosive attack, while using little or no hexavalent chromium. The present invention fulfills this need, and further provides related advantages by retaining the same efficacy as chromate.
The foregoing examples and limitations associated therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon reading of the specifications and study of the drawings.